1. Field of the Invention
The present invention relates to an EL (Electro-luminescence) display device formed by fabricating a semiconductor device (a device utilizing a semiconductor thin film; typically a thin film transistor) on a substrate, and an electronic device having such an EL display device as a display portion.
2. Description of the Related Art
A technique for forming a thin film transistor (hereinafter referred to as the TFT) on a substrate has been significantly improved in these days, and development for application thereof to an active matrix type display device has continued. In particular, a TFT utilizing a polysilicon film has a field effect mobility higher than that obtainable in a TFT utilizing a conventional amorphous silicon film, thereby realizing an operation at higher speed. Thus, it becomes possible to control pixels by a driving circuit formed on the identical substrate on which the pixels are formed, which is different from the conventional case in which the pixels are controlled by a driving circuit formed in the outside of the substrate.
Such an active matrix type display device has drawn much attention thereto since the device can realize various advantages, such as reduction in manufacturing cost, down-sizing of a display device, an improved yield, a reduced throughput or the like, by fabricating various circuits and devices on one and the same substrate.
In an active matrix EL display device, each pixel is provided with a switching device made of a TFT, and a driving device for controlling a current is activated by the switching device to cause an EL layer (more strictly speaking, a light emitting layer) to emit light. The EL display device is disclosed, for example, in Japanese Laid-open Patent Publication No. Hei 10-189252.
Thus, the present invention is intended to provide an inexpensive EL display device capable of displaying an image with high definition. Furthermore, the present invention is also intended to provide an electronic device having a high recognizability of a display portion by utilizing such an EL display device as the display portion.
The present invention will be described with reference to FIG. 1. In FIG. 1, reference numeral 101 denotes a substrate having an insulating surface. As the substrate 101, an insulating substrate such as a quartz substrate can be used. Alternatively, various kinds of substrate, such as a glass substrate, a semiconductor substrate, a ceramic substrate, a crystallized substrate, a metal substrate, or a plastic substrate, can be used by providing an insulating film on a surface thereof.
On the substrate 101, pixels 102 are formed. Although only three of the pixels are illustrated in FIG. 1, a higher number of pixels are actually formed in matrix. In addition, although one of these three pixels will be described below, the other pixels have the same structure.
In each of the pixels 102, two TFTs are formed; one of them is a switching TFT 103, and the other is a current control TFT 104. A drain of the switching TFT 103 is electrically connected to a gate of the current control TFT 104. Furthermore, a drain of the current control TFT 104 is electrically connected to a pixel electrode 105 (which in this case, also functions as a cathode of an EL element). The pixel 102 is thus formed.
Various wirings of the TFT as well as the pixel electrode can be formed of a metal film having a low resistivity. For example, an aluminum alloy film may be used for this purpose.
Following the fabrication of the pixel electrode 105, an insulating compound 106 (hereinbelow, referred to as the alkaline compound) including an alkali metal or an alkaline-earth metal over all of the pixel electrodes. Note that the outline of the alkaline compound is indicated by a dotted line in FIG. 1. This is because the alkaline compound 106 has a thickness which is as thin as several nm, and it is not known whether the compound 106 is formed as a layer or in an island-shape.
As the alkaline compound, lithium fluoride (LiF), lithium oxide (Li2O), barium fluoride (BaF2), barium oxide (BaO), calcium fluoride (CaF2), calcium oxide (CaO), strontium oxide (SrO), or cesium oxide (Cs2O) can be used. Since these are insulating materials, short-circuiting between the pixel electrodes does not occur even when the alkaline compound 106 is formed as a layer.
It is of course possible to use as a cathode a known conductive material as an MgAg electrode. However, in this case, the cathode itself has to be selectively formed or patterned into a certain shape in order to avoid short-circuiting between the pixel electrodes.
After the alkaline compound 106 is formed, an EL layer (Electro-luminescence layer) 107 is formed thereover. Although any known material and/or structure can be employed for the EL layer 107, a material capable of emitting white light is used in the present invention. As the structure, only a light emitting layer providing a field for recombination may be employed for the EL layer. If necessary, an electron injection layer, an electron transport layer, a hole transport layer, an electron blocking layer, a hole device layer, or a hole injection layer may be further layered. In the present specification, all of those layered intended to realize injection, transport or recombination of carriers are collectively referred to as the EL layer.
As an organic material to be used as the EL layer 107, either a low-molecule type organic material or a polymer type (high-molecule type) organic material can be used. However, it is desirable to use a polymer type organic material that can be formed by an easy formation technique such as a spin coat technique, a printing technique or the like. The structure illustrated in FIG. 1 is of the color display scheme in which an EL layer for emitting white light is combined with a color filter.
Alternatively, a color display scheme in which an EL layer for emitting blue or blue-green light is combined with fluorescent material (fluorescent color conversion layer; CCM), or another color display scheme in which EL layers respectively corresponding to RGB are overlaid one upon another to provide color display, can also be employed.
Over the EL layer 107, a transparent conductive film is formed as an anode 108. As the transparent conductive film, a compound of indium oxide and tin oxide (referred to as ITO), a compound of indium oxide and zinc oxide, tin oxide or zinc oxide can be used.
Over the anode 108, an insulating film as a passivation film 109 is provided. As the passivation film 109, a silicon nitride film or a silicon oxynitride film (represented as SiOxNy) is preferably used. Although a silicon oxide film may be used, an insulating film with as low an oxygen content as possible is preferred.
The substrate fabricated up to this stage is referred to as an active matrix substrate in the present application. More specifically, the substrate on which a TFT, a pixel electrode electrically connected to the TFT, and an EL element (a capacitor made of a cathode, an EL layer, and an anode) utilizing the pixel electrode as the cathode are formed is referred to as the active matrix substrate.
Furthermore, a counter substrate 110 is attached to the active matrix substrate with the EL element being interposed therebetween. The counter substrate 110 is provided with a light shielding film 112 and color filters 113a to 113c. 
At this situation, the light shielding film 112 is provided so that a gap 111 formed between the pixel electrodes 105 is unseen from the viewing direction of an observer (i.e., from a direction normal to the counter substrate.) More specifically, the light shielding film 112 is provided to overlap (align with) the periphery of the pixel when viewed from the direction normal to the counter substrate. This is because this portion is non-emitting portion, and furthermore, electric field becomes complicated at the edge portion of the pixel electrode and thus light cannot be emitted with a desired luminance or chromaticity.
More specifically, by providing the light shielding film 112 at the position corresponding to the periphery (edge portion) of the pixel electrode 105 and the gap 111, contour between the pixels can be made clear. It can be also said that in the present invention, the light shielding film 112 is provided at the position corresponding to the periphery (edge portion) of the pixel because the contour of the pixel electrode corresponds to the contour of the pixel. Note that the position corresponding to the periphery of the pixel refers to the position aligned with the periphery of the pixel when viewed from the aforementioned direction which is normal to the counter substrate.
Among the color filters 113a to 113c, the color filter 113a is the one for obtaining red light, the color filter 113b is the one for obtaining green light, and the color filter 113c is the one for obtaining blue light. These color filters are formed at positions respectively corresponding to the different pixels 102, and thus, different color of light can be obtained for the respective pixels. In theory, this is the same as the color display scheme in a liquid crystal display device which uses color filters. Note that the position corresponding to the pixel refers to the position overlapped (aligned) with the pixel when viewed from the aforementioned direction which is normal to the counter substrate. More specifically, the color filters 113a to 113c are provided so as to overlap the pixels respectively corresponding thereto when viewed from the direction normal to the counter substrate.
Note that the color filter is a filter for improving the color purity of light which has passed therethrough by extracting light of a specific wavelength. Accordingly, in the case where the light component of the wavelength to be extracted is small, there may be disadvantages in which the light of that wavelength has extremely small luminance or deteriorated color purity. Thus, although no limitation is imposed to an EL layer for emitting white light which can be used in the present invention, it is preferable that the spectrum of the emitted white light includes emission spectrums of red, green and blue light components having as high purity as possible.
FIGS. 16A and 16B show typical x-y chromaticity diagrams of an EL layer to be used in the present invention. More specifically, FIG. 16A shows the chromaticity coordinate of light emitted from a known polymer type organic material for emitting white light. In the known material, the red color emission with high color purity cannot be realized. Therefore, yellow light or orange light is used instead of red light. Accordingly, white color obtained by adhesive color mixing seems to slightly include green color or yellow color. In addition, the respective emission spectrums of red light, green light, and blue light are so broad that it also becomes difficult to obtain monochromatic light having high purity when these light are mixed.
Accordingly, although sufficient color display can be realized even when an organic material as represented in the chromaticity diagram in FIG. 16A is used as an EL layer, it is now preferable to use as an EL layer an organic material as represented in the chromaticity diagram in FIG. 16B in order to realize brighter color display with higher purity.
The organic material as represented in the chromaticity diagram in FIG. 16B is an example in which an EL layer for emitting white light is formed by mixing organic materials capable of providing monochromatic light with high purity. In order to obtain light emission spectrums of red, green and blue colors having high color purity from a color filter, it is necessary to form an EL layer for emitting white light by mixing organic materials respectively exhibiting light emission spectrums of red, green and blue colors with high color purity. In addition, by using materials capable of providing a spectrum not only with high color purity but also with a narrow half-peak width, white color with a sharp spectrum can be reproduced. With this kind of the EL layer for emitting white light, the present invention can display a further brighter color image.
Furthermore, the color filters 113a to 113c can contain, as a drying agent, an oxide of an element in group I or II in periodic table, e.g., barium oxide, calcium oxide, lithium oxide or the like. In this case, a resin film containing a drying agent and a pigment of red, green or blue color may be used as a color filter.
Note that although not illustrated herein, the counter substrate 110 is adhered to the active matrix substrate by means of a sealing agent, so that a space designated with reference numeral 114 is a closed space.
As the counter substrate 110, it is necessary to use a transparent substrate so as not to prevent light from traveling. For example, a glass substrate, a quartz substrate, or a plastic substrate is preferably used. In addition, as the light shielding film 112, a thin film capable of satisfactorily shielding light, e.g., a titanium film, a resin film including a black-colored pigment or carbon. Similarly to the case of the above-mentioned color filters 113a to 113c, it is advantageous to provide the light shielding film 112 containing, as a drying agent, an oxide of an element in group I or II in periodic table, e.g., barium oxide, calcium oxide, lithium oxide or the like.
The closed space 114 may be filled with inert gas (noble gas or nitrogen gas), or with inert liquid. Alternatively, the closed space 114 may be filled with a transparent adhesive so as to adhere the whole surface of the substrate. Moreover, it is preferable to dispose a drying agent such as barium oxide in the closed space 114. Since the EL layer 107 is very vulnerable to water, it is highly desirable to prevent water from entering the closed space 114.
In the EL display device having the above-mentioned construction in accordance with the present invention, light emitted from the EL element passes through the counter substrate to be emitted toward an observer""s eyes. Accordingly, the observer can recognize an image through the counter substrate. In this situation, one of the features of the EL display device in accordance with the present invention is that the light shielding film 112 is disposed between the EL element and the observer so as to conceal the gap 111 between the pixel electrodes 105. Thus, the contour between the pixels can be made clear, thereby resulting in an image display with high definition. This advantage can be obtained due to the light shielding film 112 provided at the counter substrate 110. When at least the light shielding film 112 is provided, this advantage can be obtained.
Furthermore, the light shielding film 112 and the color filters 113a to 113c are disposed at the counter substrate 110, and the counter substrate 110 also functions as a ceiling substrate for suppressing deterioration of the EL element. When the light shielding film 112 and the color filters 113a to 113c are disposed at the active matrix substrate, additional film-formation and patterning steps are required, whereas the number of fabrication steps for the active matrix substrate can be suppressed in the case where they are provided at the counter substrate, although additional film-formation and patterning steps are required.
Furthermore, the structure in accordance with the present invention in which the counter substrate 110 is provided with the light shielding film 112 and the color filters 113a to 113c and adhered to the active matrix substrate by means of the sealing agent has features common to the structure of a liquid crystal display device. Accordingly, it is possible to fabricate the EL display device of the present invention with most of an existing manufacturing line for liquid crystal display devices. Thus, an amount of equipment investment can be significantly reduced, thereby resulting in a reduction in the total manufacturing cost.
Thus, in accordance with the present invention, an inexpensive EL display device capable of displaying an image with high definition can be obtained. Furthermore, the present invention can also provide an electronic device having a high recognizability of a display portion by utilizing such an EL display device as the display portion.